Diamond is an allotrope of carbon exhibiting a crystallographic network comprising exclusively of covalently bonded, aliphatic sp.sup.3 hybridized carbon atoms arranged tetrahedrally with a uniform distance of 1.545 .ANG. between atoms. Diamond is extremely hard having a Mohs hardness of 10. It exhibits four times the thermal conductivity of copper and it is electrically insulating. Its hardness and thermal properties are but two of the characteristics that make diamond useful in a variety of industrial components. Initially non-gem quality natural diamonds were used in a variety of abrasive applications but with the invention of synthetic diamonds by high pressure/high temperature techniques, a spectrum of additional products have found favor in the marketplace. However, the requirement of high pressure and high temperature has been a limitation in preventing extensive usage of synthetic diamonds.
Recent industrial efforts directed toward the growth of diamonds at low pressures has dramatically increased the feasibility of using diamonds in various industrial applications. Low pressure growth of diamond has been dubbed "chemical vapor deposition" or "CVD" in the field. In the CVD process diamonds are grown at low pressures from hydrocarbon gases in the presence of atomic hydrogen. Many methods have been disclosed for growing diamonds metastably and generally these methods differ from each other by the way in which atomic hydrogen, a key reactant, is generated and transported within the system. One of such methods called a "filament method" involves the use of a mixture of a gaseous carbon compound such as methane and hydrogen. The gas mixture is introduced into a substantially evacuated reaction chamber via a quartz tube located just above a hot filament. The hydrogen from the gas mixture dissociates at the filament surface into atomic hydrogen which then reacts with the carbon compound to form condensable carbon radicals including elemental carbon onto a heated substrate.
Another method involves a plasma discharge in addition to the filament. The plasma discharge serves to increase the nucleation density and growth rate and, it is believed, to enhance formation of diamond films as opposed to discrete diamond particles. Of the plasma systems that have been utilized in this area, there are three basic systems. One is microwave plasma system, the second is an RF (inductively or capacitively coupled) plasma system and the third is a d.c. arc plasma system.
When a free standing diamond film is required, the underlying substrate may be etched out by conventional etching techniques, such as exposing the substrate to the etching acids. For a general summary of various diamond deposition methods including CVD methods, reference is made to Bachmann, et al., Diamond Thin Films, Chemical & Engineering News, 67(20), 24-39 (May 15, 1989), incorporated herein by reference.
Various problems have been encountered in producing free standing CVD diamond deposition films. For example, a long etch time, up to ten days, is required to etch away the underlying substrate, such as a solid molybdenum mandrel of about 0.102 centimeters in diameter and about 15.24 centimeters in length, upon which a diamond tube is formed. In addition to the long etch times, it is also found that the reaction products formed at the interface between the mandrel being etched and the deposited diamond cause swelling and cracking of the diamond tube.